The field of this invention is that of thermostat metals and the invention relates more particularly to low cost, high flexivity thermostat metals particularly adapted for use in corrosive environments.
Thermostat metals basically comprise a layer of metal of relatively high coefficient of thermal expansion bonded to a layer of metal of relatively lower coefficient of thermal expansion so that the resulting composite materials flex in response to temperature change. Such thermostat metals have previously incorporated stainless steel materials for various purposes and have used stainless steels as well as other metal claddings or platings for providing the thermostat metals with corrosion resistance properties.
In that regard, commercially available austenitic stainless steels are known to have particularly desirable corrosion resistance properties. They have also been generally considered to have relatively high coefficients of thermal expansion. Accordingly, they have been used the high expansion sides of thermostat metals where their high expansion properties have been intended to cooperate with the characteristics of the other metal layers in the composites to provide the thermostat metals with high flexivity while the iron-chromium-nickel alloys of the austenitic stainless steels have provided excellent corrosion protection for the high expansion side of the thermostat metals.
Usually materials other than stainless steels such as invar or the like have been used as the low expansion materials in composite thermostat metals and, when additional corrosion resistance properties have been required on the outer surface of the low expansion sides of thermostat metals, some difficulty has been encountered. For example, where thin stainless steels have been used for protecting the low expansion sides of the thermostat metal against corrosion, ferritic stainless steels have been used. The iron-chromium alloys of the ferritic stainless steels are known to provide less substantial corrosion resistance properties than austenitic stainless steels and usually require processing in controlled neutral or reducing atmospheres. Use of such ferritic materials also tend to involve some parts manufacturing problems or parts welding problems due to the occurrence of chromium oxides in the ferritic stainless steels which can interfere with welding or cause an undesirable degree of tool wear. If ferritic stainless steels with titanium contents are selected to improve weldability, the occurrence of titanium oxides also tend to cause undesirable tool wear. However, the ferritic steels are used because they are generally considered to have significantly lower coefficients of thermal expansion. Thus, the ferritic materials have been used because they had been considered to have restricted effect in limiting thermostat metal flexivity even though their corrosion resistance properties and processing requirements have been less than fully desirable.